Plasma-based illumination source systems, such as laser-produced plasma (LPP) sources, laser-sustained plasma (LSP) sources, laser-driven light sources (LDLS), or discharge-produced plasma (DPP) sources, are often used to generate soft x-ray, extreme ultraviolet (EUV), and vacuum ultraviolet (VUV) wavelengths of illumination (e.g. wavelengths around 120 nm or shorter) for applications such as defect inspection, photolithography, or metrology. The illumination may be emitted by a plasma that is generated at or near a site where target material (e.g. xenon, tin, or lithium) is deposited and irradiated by an excitation source, such as a laser. Illumination emanating from the plasma may be collected and directed and/or focused along an illumination delivery path.
Plasma-based light sources, such as EUV, used for lithography, mask inspection and metrology also suffer from debris generated as a by-product of the plasma formation process, which can include high energy ions and neutrals, atomic clusters, condensable vapor, particles, etc. These undesired by-products can cause serious damage and shorten lifetime of optical elements placed inside the EUV source vacuum chamber, e.g., laser entrance window, EUV collector, metrology windows and mirrors, detectors, etc.
There is a continuing need for improved mechanisms for protecting illumination source elements in plasma-based illumination generation or source systems from debris.